German Patent Application No. DE 101 39 142 describes an exhaust-gas treatment system of an internal combustion engine, in which an SCR catalytic converter (selective catalytic reduction) is used for reducing the NOx emission, which reduces the nitrogen oxides NO and NO2 to Nitrogen N2 using the reducing agent ammonia. The ammonia is obtained in a hydrolysis catalytic converter from a urea-water solution. The hydrolysis catalytic converter, situated in front of the SCR catalytic converter in the direction of flow, converts the urea contained in the urea-water solution to ammonia and carbon dioxide with the aid of water. To ensure exact metering, there is a provision to ascertain the concentration of the urea-water solution.
German Patent Application No. DE 103 16 184 (not prepublished) describes a method for metering the urea-water solution into the exhaust-gas stream of an internal combustion engine, in which the urea-water solution is carried in at least one component and a measure for the temperature of the component is detected. The measure for the temperature is compared to a temperature threshold value corresponding to the freezing temperature of the urea-water solution. A freezing cycle counter records the number of freezing processes of the urea-water solution, which subject the components to mechanical stress. When a specified counter reading is exceeded, a diagnostic instruction may be given for checking the components. An existing follow-up control, which is active after the internal combustion engine has been switched off, makes it possible to check whether the component is still filled with the urea-water solution even after the internal combustion engine has been switched off.
Prior to being injected into the exhaust-gas region of the internal combustion engine, the ammonia can be obtained in an external ammonia source from ammonium carbamate for example. The preparation of the ammonia either in an external ammonia source or in a hydrolysis catalytic converter requires that a certain temperature range obtains. The operating temperature of a hydrolysis catalytic converter for example lies in a range of 180° C.-250° C. Hence, when the internal combustion engine is put into operation beginning with a cold start, the ammonia required for the reduction reaction in the SCR catalytic converter is generally not yet available. In this operating state there is consequently an increased NOx emission behind the as yet ineffective SCR catalytic converter.
The present invention is based on the objective of providing a method for metering ammonia into the exhaust-gas region of an internal combustion engine as well as a device for implementing the method, which allow for the adherence to an NOx emission that is as low as possible.